BERLIN » Ukrainian President Petro Poroshenko accused Russia on Thursday of invading his nation, at about the same time that a separatist rebel leader admitted his forces are made up of Russian fighters, including active and retired Russian troops.

This accusation of invasion involved events in what’s now being called a second front in the fight in Ukraine’s southern territory, Donetsk. Ukrainian troops guarding what had in recent weeks been a calm piece of the nations’ shared border near the Black Sea came under rocket fire from Russia, then faced "attack by two columns, including tanks," a top military spokesman said Thursday in Kiev.

While it was hardly the first evidence this year of a hostile Russian military presence in Ukraine, it was seen as something of a tipping point. Poroshenko canceled a scheduled "working trip" to Turkey, noting, "The president must stay in Kiev today."

He went on to add that his actions were necessary "because of an introduction of Russian troops into Ukraine."

At a news conference Thursday in Washington, President Barack Obama blamed Russia for the violence in eastern Ukraine but stopped short of describing the latest escalation as an invasion.

"And this ongoing Russian incursion into Ukraine will only bring more costs and consequences for Russia," he said. "What we’re doing is to mobilize the international community to apply pressure on Russia."

Obama said he’d encourage U.S. allies at next week’s NATO summit to take additional steps to resolve the crisis diplomatically. He also announced that Poroshenko will visit the White House next month.

Even as Ukrainian officials were claiming Russian troops had invaded, the leader of their opposition, the prime minister of the so-called Donetsk People’s Republic, Alexander Zakharchenko, said in a Russian television interview on Rossiya 24 that there were 3,000 to 4,000 Russian volunteers fighting in his force.

He said the majority of those who’d volunteered to fight were still there. Some, he said, had lost their lives.

"Among the volunteers who come here are many former career military. They are fighting with us and see it as their duty," he said. "Apart from that, I’m telling you, we have current military personal in our ranks who preferred to spend their vacations not on beaches but on our side.

"We never hid the fact that there are many Russians among us, without the help of who it would have been very difficult for us to get to where we are today," he said.

While the conflict has been constant in the region since March, Thursday’s information appeared to represent another stage in the fight that’s between Ukraine and what were said to be, up until Thursday, Ukrainian pro-Russian separatists.

The last such point was the shooting down of Malaysian Airlines Flight 17 in July, which killed nearly 300 civilians who happened to be flying over the region. The passenger jet was shot down by what Ukrainian and Western officials think was a Russian anti-aircraft missile system supplied to rebel fighters.

In comments posted Thursday by the Ukrainian Crisis Media Center website, Poroshenko noted that his nation needs global support in this crisis.

"The world has to comment on the severe aggravation of the situation in Ukraine today," he said.

Stephen Long, an international security expert at the University of Richmond in Virginia, noted that it’s possible Russian soldiers could decide to be on vacation while fighting in Ukraine, "though if that were the case it would be hard to imagine that they could do that without the permission and support of their nation. And generally when troops go on vacation, they don’t get to take their tanks."

In fact, Long noted, Russian President Vladimir Putin’s refutations to accusations of Russian involvement in the fighting have tended to fall into the unbelievable or even humorous category from the beginning.

"He’s not even making a good-faith effort to come up with explanations that anyone in the United States or Europe could believe," he said. "No matter what Russia calls it, it’s been obvious for a long time that Russia is actively involved."

Indeed, accusations of a Russian invasion from members of the Ukrainian military and government are hardly a first. There were similar accusations last week, and last month, and at some level pretty much every week and sometimes daily since March, when Russian soldiers, or the now-infamous "little green men," spread out across the Ukrainian Crimean peninsula in the weeks that led up to what’s now a deeply disputed referendum.

That referendum, which Russia insisted showed that 96 percent of Crimeans wanted to join Russia, led to Russia annexing the region. Ukraine and Western nations haven’t accepted the annexation. Last week, meeting with Poroshenko, German Chancellor Angela Merkel noted the Crimean annexation was invalid.

"It was in violation of the territorial integrity of Europe," she said.

This week though, in making the claims of Russian involvement, the Ukrainian president is able to cite new evidence. Ukrainian forces have captured Russian military hardware, from guns to tanks. They’ve arrested numerous "separatist fighters" who have only Russian passports. This week, 10 active Russian military paratroopers were arrested in eastern Ukraine. Their defense: They were lost and had no idea they were in Ukraine.

News reports from the front note that Ukrainian soldiers there today are finding their foes are almost entirely Russian.

As Russian involvement has become more brazen in recent weeks — since separatist forces appeared to be losing ground last month — former Soviet Union ally nations such as Poland and Lithuania have grown noticeably nervous. In the United Nations Security Council, Lithuania called the events of the day an "invasion by the armed forces of the Russian Federation."

Ukrainian military spokesman Col. Andriy Lysenko in Kiev described the attack in the south of Ukraine, "which entered Ukrainefrom Veselo-Voznesenka and Maksymovo of the Rostov Oblast in Russia."

At a news conference, he said the attack followed the missile barrage, and began at 12:30.

"Our border servicemen and guardsmen retreated, as they did not have heavy equipment," he said. He added that they’d retreated from the Ukrainian town of Novoazovsk on orders, and "both Russian columns entered" soon after.

Lysenko said the forces now attacking Ukraine were Russian, as official Russian troops in the past week "had replaced … the regiments of local ‘rebels’ and Cossacks, who were unable to resist" Ukrainian forces.

Beyond Russian fighters, he said, Ukraine has information that "Russian medical personnel arrived at the Krasnodon Central City Hospital with medical equipment, after which wounded and killed mercenaries were mass-delivered to the hospital."

He noted that presence "is yet another piece of evidence to the fact that regular Russian servicemen are involved in the conflict. We have reported on the instances of Russian aggression numerous times."